Auto merge of #109888 - Mark-Simulacrum:balanced-compression, r=pietroalbini

Remove optimal xz settings from CI

This is a companion PR to rust-lang/promote-release#58, which moves the relevant optimal code to rust-lang/promote-release. As mentioned in the comments of that PR, this is expected to cut CI costs (and time, though predominantly felt on fast builders) and reduce wasted resources due to in-practice single-threaded compression not using the full 8+ vCPU builders we have available.

This probably shouldn't land before that PR + a simpleinfra change to enable the recompression of xz artifacts. But if it does land, it's just a matter of a few nightlies with slightly larger artifacts, so not a big deal.

r? `@pietroalbini`

Author: bors